Hydraulic fracturing is a common stimulation technique used to enhance production of fluids from subterranean formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant is injected into the wellbore at pressures sufficient to create or enlarge a fracture in the reservoir. The proppant is deposited in the fracture, where it remains after the treatment is completed. The proppant serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the wellbore through the fracture. Because well productivity depends on the ability of a fracture to conduct fluids from the formation to the wellbore, fracture conductivity is an important parameter in determining the degree of success of a hydraulic fracturing treatment.
Fracture conductivity may be reduced by small proppants or fines. In fracture conductivity testing using proppants of the prior art confined between sandstone cores, embedment of proppant into the core is frequently observed after exposure to elevated stress. In the process of embedment, spalling of fines from the rock is displaced into the proppant pack. Proppant pack conductivity damage from embedment results in loss of proppant pack width as the proppant embeds into the rock and proppant pack pore throats are plugged by displaced formation fines. The pack permeability is thereby reduced.
A second source of fines results from proppant crushing. Such fines are generated at the fracture-face to proppant pack interface as in situ closure stresses acting upon the fracture cause failure of the proppant, the formation rock, or both. Such stresses may cause the proppant to be compressed together such that fines are generated from the proppant pack and/or reservoir matrix. Further, fines composed of formation material (e.g., shale, sand, coal fines, etc.) may present similar problems and may be produced, for example, within the fractured formation due to stresses and forces applied to the formation during fracturing.
Proppant packs containing sand with a deformable proppant substantially reduce proppant crushing. Such proppant packs are disclosed in U.S. Pat. Nos. 6,059,034 and 6,330,916, herein incorporated by reference. In addition to sand, such proppant packs contain deformable additives which act as a cushion and minimize the point stresses applied to the proppant and limit crushing of the sand. However, at elevated stress levels, the permeability and porosity levels of such proppant packs are compromised by embedment and spalling.
Alternate methods of reducing proppant pack damage and minimizing fines generation at increased stress levels have therefore been sought in order to increase the productivity of subterranean reservoirs.